During an eruption, lava leaving a source (volcano, vent, or crater) travels across the surface and either reaches the ocean or eventually stops. The hardened surface produced after the lava has completely cooled and solidified isn’t necessarily that interesting. It can have an interesting look to it, but it’s not really worthwhile. However, what can be even interesting are the lava tubes that can be formed after long-lasting volcanic events.
Lava flow and lava tubes
During a volcanic event, lava flows will travel from the source. As the lava flows across the ground, it quickly forms a crust on the surface because the surface quickly cools. The lava underneath the crust is insulated by the crust and maintains most of its temperature. It will continue to flow beneath the crust and will push upwards on the crust, thereby expanding it and enabling the crust to thicken. Streams of lava will travel either through surface channels, which may have been created by other eruptions, or in chambers formed and expanded upon beneath the crust that formed. Once the lava flow has ceased and all or most of the lava flows through the channels and chambers, long tubes or cave systems will be left over. The tubes or caves are known as lava tubes or lava tube caves. Lava flow has the potential to create tubes or caves if the right conditions are met, the flow is long-lasting, and the lava does not stagnate or solidify but exits the tube and leaves it empty. The tubes themselves can be found in many locations around the world but are typically formed by basaltic lava flows which are less viscous and faster moving.
Formation of lava tubes
Lava tubes are created in one of two ways. The first method of lava tube formation is through the ‘roofing’ of the surface channels and occurs in one of three ways. As lava flows through a surface channel, the flow rate will determine which type of roofing occurs. If the lava flow is slow and at a steady rate, the lava on surface of the channel will steadily begin to crust over. This development happens at the edges of the channel spreading inward and spreads downward in the direction that the lava is flowing. In medium flow rates, the lava on the surface will cool enough in some areas to form slabs of crust that will travel down the channel as if they are rafts floating on water. At some point the floating slabs will jam up at a constriction in the channel. When this happens, the ‘log jam’ of crust slabs will begin to weld together into a solid roof. Should the lava flow rate be turbulent or very fast moving, the overflow from the channel will be splattered onto the sides of the channel and start forming a levee. As the levee builds up, it begins to arch over the channel forming a roof. In all three methods of roofing, should the lava flow cease and the channel be emptied of lava, the result in each case will be the formation of a lava tube. The tubes may be used again by further volcanic activity or could be covered over by new lava flows and buried.
The second method of lava tube formation involves lava simply overflowing from a source and traveling over the surface. Lava moving from its source without a channel will be a sheet of lava in a ‘lobe’ shape or almost a tear drop shape (from a birds-eye view) because it expands in all directions away from its source as it travels. So the thinner end is closer to the source and the larger sheet of lava in the lobe shape is away from the source. The surface of the lava will also form a crust and expand vertically as well as horizontally as the lava pressure builds. When the flow continues, at some point the pressure will become too much and the crust will rupture in one or more locations. When this happens, one or more new lobes will be formed from the lava that exits through the breach in the crust. This process of continuously forming new lobes from breaches in the crust will continue, and the lava flow will branch out from the source.
One of two things can happen with the lava flow, it will cease or the flow will continue. Should the flow cease, then the lava will be drained out of the crusted lobes and continue to drain out of all the lobes that have formed along the way. This can create a system of broad but shallow caves that form from the chambers inside the crusted lobe that have been drained. If the lava flow continues, then eventually the flow will consolidate into tube flows that will continue to feed new lobes further away from the source. The surrounding areas of lava flow will solidify. This can also form lava tubes if the lava flow ceases at some point and the lava drains from the tubes.
A very good description and diagrams representing the three formations of ‘roofing’ as well as sub-crustal drainage can be found on the Volcano Speleology website which is part of the International Union of Speleology website (speleology is the study of caves).
Length of lava tubes and features
In either case of formation, the resulting lava tubes or lava tube caves can be either a short length or very long, depending on the extent of the formation. Lava tubes can range from several hundred feet in length all the way up to miles in length, which can be quite common. A lava tube in Hawaii is over 50 kilometers long, while some in Australia are even three times that length at about 150 kilometers. The tubes can also be just a few or many meters wide, and can vary along their length. The lava tube caves can extend for quite a distance as well, but will consist of numerous individually connected caves/chambers and not one continuous tube.
Inside of the tubes and caves, the lava can sometimes form different features similar to other cave systems of the world. Leftover lava can drip from the walls of the lava tubes and dry, leaving lava stalactites and stalagmites on the roof and floor of the tube respectively. The walls can also start peeling or tear off in some areas. Sometimes the lava flow can create wave-like designs on the sides of the tube or different features on the floor of the lava tube. The lava tubes and different formations inside of them make them some of the most interesting physical features that result from volcanic activity.